Summary

This report shows maps and time series data of potential harmful algal blooms (HAB) in several of Denver Water’s reservoirs. The reservoirs in this report were selected by Denver Water as key locations where HABs could cause increased water treatment requirements or potential public health concerns.

Map of all reservoirs in the report.

HABs may be detected through the use of remote sensing algorithms based on data from the Sentinel 2 Level 1-C Top of Atmosphere satellite. There are two key metrics shown in this report that relate to HAB presence:

NIR:Red: the near infrared (NIR) to red band ratio. The higher NIR:Red values relate to increasing algae biomass on the water surface (Tebbs et al., 2013).The NIR:Red ratio is close to zero for clear water and increases with algae concentrations and values greater than 1 indicates algae presence (Tebbs et al., 2013). NIR:Red is calculated in equation 1 below.

where R represents the top of atmosphere reflectance (0–1) for a given band (e.g., \(R_{NIR}\) is the reflectance value for the NIR band).

FAI-NDWI: This algorithm uses the floating algae index (FAI) and the normalized difference water index (NDWI). This algorithm detects potential cyanobacterial blooms (Oyama et al., 2015). First, the FAI differentiates between clear water and algae using a threshold of 0.05. The FAI considers values greater than or equal to 0.05 to be algae, while those below are clear water. Next, the NDWI partitions algae (i.e., pixels with FAI ≥ 0.05) into cyanobacteria and non-cyanobacteria blooms using a threshold of 0.63. Values greater than or equal to this threshold are considered probable cyanobacterial blooms while those below are not. This algorithm is described in equations 2 and 3 below.

Dense algae on the water surface can yield taste and odor issues, additionally cyanotoxins may be present in the water with cyanobacteria presence. So remote sensing is a valuable tool to map algal blooms and potential HABs because the data are continuous, frequent, and freely available. While remote sensing cannot determine toxicity, it offers near real time results and provides drinking water managers a tool to understand the seasonality and presence of algal blooms to assist in decision making.

Key Results

Detection points were placed in four different areas around each reservoir. Points are labeled edge, near edge, and open water to indicate the distance from shore. This is to help differentiate the satellite results during times of low water or high water.

Table XX below describes results at each reservoir using the algorithms above. This table highlights data from the period XX to YY. The other tables and figures in this report show results for the entire season so far.

TABLE HERE

Antero Reservoir

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The following images are maps showing potential harmful algal blooms in Antero Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Antero Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

Bambei-Walker Reservoir

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The following images are maps showing potential harmful algal blooms in Bambei-Walker Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Bambei-Walker Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

Chatfield Reservoir

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The following images are maps showing potential harmful algal blooms in Chatfield Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Chatfield Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

Cheesman Reservoir

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The following images are maps showing potential harmful algal blooms in Cheesman Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Cheesman Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

Dillon Reservoir

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The following images are maps showing potential harmful algal blooms in Dillon Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Dillon Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

Dunes Reservoir

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The following images are maps showing potential harmful algal blooms in Dunes Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Dunes Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

Gross Reservoir

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The following images are maps showing potential harmful algal blooms in Gross Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Gross Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

Marston Reservoir

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The following images are maps showing potential harmful algal blooms in Marston Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Marston Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

Platte Canyon Reservoir

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The following images are maps showing potential harmful algal blooms in Platte Canyon Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Platte Canyon Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

Ralston Reservoir

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The following images are maps showing potential harmful algal blooms in Ralston Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Ralston Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

Strontia Springs Reservoir

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The following images are maps showing potential harmful algal blooms in Strontia Springs Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Strontia Springs Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

Tanabe Reservoir

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The following images are maps showing potential harmful algal blooms in Tanabe Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Tanabe Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

Welby Reservoir

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The following images are maps showing potential harmful algal blooms in Welby Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Welby Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

Williams Fork Reservoir

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The following images are maps showing potential harmful algal blooms in Williams Fork Reservoir from 2020-04-01 and 2020-11-01.

NIR:Red mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2020-04-01 to 2020-11-01 study period. Darker colors indicate higher FAI-NDWI exceedances. MON in the site name means monitor.

At Williams Fork Reservoir timeseries data is processed for 4 locations as shown in Figure 3 below. At each monitoring point, 3 different timeseries are collected at 3 sites spaced 10m apart. These additional points are intended to minimize false positive threshold exceedances since vegetation at the water’s edge can confuse both algorithms.

NIR:Red time series from 2020-04-01 to 2020-11-01 study period. The blue dashed line is at NIR:Red = 1.”

References

Oyama, Y., Matsushita, B., & Fukushima, T. (2015). Distinguishing surface cyanobacterial blooms and aquatic macrophytes using Landsat/TM and ETM+ shortwave infrared bands. Remote Sensing of Environment, 157, 35–47. https://doi.org/10.1016/j.rse.2014.04.031

Tebbs, E. J., Remedios, J. J., & Harper, D. M. (2013). Remote sensing of chlorophyll-a as a measure of cyanobacterial biomass in Lake Bogoria, a hypertrophic, saline–alkaline, flamingo lake, using Landsat ETM+. Remote Sensing of Environment, 135, 92–106.